A wide variety of medical systems are implanted within patients to provide a therapy to and/or monitor a physiologic condition of a patient. These implantable medical systems may include an implantable medical device (IMD) and one or more implantable medical leads to deliver therapy to or monitor conditions of a number of organs, nerves, muscles or tissues of the patient, such as the heart, brain, stomach, spinal cord, pelvic floor or the like.
Occasionally, patients that have implantable medical systems may benefit from a medical procedure that may generate an interfering signal. For example, a patient may benefit from a magnet resonance image being taken of a particular area of his or her body. Magnetic resonance imaging (MRI) is a technique for imaging portions of the body of the patient for purposes of medical diagnosis. During an MRI procedure, the patient is exposed to magnetic and radio frequency (RF) fields to obtain images of a portion of the body. In particular, the patient is exposed to a strong static (i.e., non-varying) magnetic field that is typically always present around the MRI device whether or not a procedure is in progress. In the presence of the strong static magnetic field, a number of gradient (i.e., time-varying) magnetic fields and RF fields are applied during the MRI procedure to obtain the desired images. The magnitude, frequency or other characteristic of the various fields applied during the MRI procedure may vary based on the type of device producing the fields or the type of scan being performed.
Exposure of the implantable medical system to the various fields generated by the MRI device may result in undesirable operation of the implantable medical system. In some instances, the gradient magnetic fields or the RF fields may induce a current or voltage on the leads of the implantable medical system. The current or voltage induced on the leads may interfere with the ability of the IMD to properly sense cardiac signals of the heart of the patient. For example, current or voltage induced on the lead by the gradient magnetic or RF fields may cause the IMD to incorrectly sense a cardiac signal when one is not present or to fail to sense a cardiac signal when one is present. Such interference may result in the IMD delivering therapy when it is not desired or withholding therapy when it is desired.